It's flown 2.3 billion miles in search of its prey: less than a thimbleful of microscopic particles shed by a comet, the most primitive stuff known in the solar system.

Friday morning, NASA's Stardust spacecraft will pounce.

Starting about 11:40 a.m., the spacecraft will zip within 200 miles of Comet Wild 2 and extend a robotic arm to collect comet dust in a trap so light and airy, it looks like solid smoke.

The dust, by the way, will slam into the collector at six times the speed of a rifle bullet.

It'll all be over in eight minutes. Then, if things go well, Stardust will head for home and a 2006 landing in the Utah desert, delivering its cargo to scientists who will pore over the grains for decades to come.

``It will be kind of halfway between elation that we've gotten there and knuckle-biting,'' said Scott Sandford, a space scientist at NASA/Ames Research Center at Moffett Field and one of the co-investigators for the mission. ``We'll all obviously heave a sigh of relief when we hear this thing communicating after the encounter.''

First to try collection

Researchers have been hankering for a piece of a comet for 25 years. Although previous missions have flown close to comets, they were not designed to bring anything back.

Stardust is the first to try. Launched four years ago, it's one in a series of relatively low-budget Discovery missions, each costing less than $150 million.

Why the fascination with comets?

``Most of the material we can study in the solar system is planetary stuff -- things from the moon, Mars, whatever,'' Sandford said. ``Those things have been savagely reconstructed since the solar system formed. You can't find a rock on the Earth that is a witness, as a rock, to the birth of the Earth. You have to go some place where the material was parked for a long time and left relatively unchanged.''

Comets originate in just such a place, far beyond the orbits of the planets. They're balls of ice, rock and dust, leftovers from the formation of the solar system some 4.6 billion years ago.

Space is also full of stardust -- small, rocky grains thrown off by dying stars and recycled endlessly into new stars, planets, perhaps even living things. The Stardust spacecraft will also collect some of this stuff and bring it back for study.

NASA/Ames is one of the world's foremost centers for this type of research. In a pair of small laboratories there, Sandford and his colleagues create bits of artificial stardust and then torture it -- alternately freezing it, frying it and blasting it with radiation to see how it evolves in the harsh space environment over billions of years.

The spacecraft already has survived some scary times, including massive solar flares that could have disrupted its electronics. The camera it uses for navigation was contaminated during launch, blurring its vision, but controllers managed to burn most of the contamination off and restore the camera's sight.

Precious cargo

Now, Sandford said, the big worry is that the spacecraft might hit a chunk of rock when it attempts to scoop dust from Wild 2.

``One of these grains is a little world in and of itself,'' Sandford said. ``You can pry particles apart on a really tiny scale,'' sputtering away the atoms one at a time, devoting months of study to a single grain.

The dust will be stored, along with moon rocks and meteorites, at the National Aeronautics and Space Administration's Johnson Space Flight Center in Houston. From there, it'll be parceled out to scientists. ``Long after I'm gone,'' Sandford said, ``people will still be learning things about comets from these samples.''

For information on the Stardust mission, see http:// stardust.jpl.nasa.gov/index.html. For more on Sandford's work -- including a photo of him braving Antarctic cold in a swim suit -- see http://stardust.jpl.nasa.